Piston with keystone second ring groove for high temperature internal combustion engines

ABSTRACT

A piston for a high temperature internal combustion engine is provided. The piston includes an upper wall, base wall, outer rib, and inner rib defining a cooling chamber therebetween, and a plurality of ring grooves formed in the outer rib. Only the second ring groove is formed with the keystone cross-section, and all of the other ring grooves are formed with the conventional rectangular cross-section. Thus, the piston can be formed with low manufacturing costs and can also provide exceptional performance when used in high temperature combustion engines, wherein the temperature at the first ring groove is greater than 280° C., and thus prevents carbon from depositing or burns off any carbon deposits, but the temperature at the second ring groove is between 200° C. and 280° C., in which case carbon deposits can form and cause the piston ring to stick.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to pistons for internal combustion engines, and methods for manufacturing the same.

2. Related Art

Pistons of internal combustion engines, such as heavy duty diesel pistons, are exposed to extremely high temperatures during operation, especially along the upper crown of the piston. Such pistons typically include a plurality of ring grooves formed along the outermost surface and each containing a piston ring which slides along the cylinder wall as the piston reciprocates. The piston rings assist in the transfer heat away from the piston to the cylinder wall. The piston rings can also seal the combustion chamber and limit oil consumption. Oftentimes, during engine operation, the temperature of the first (uppermost) ring groove exceeds 200° C., in which case carbon deposits tend to form along the ring groove or piston ring. Over time, the carbon deposits can cause the piston ring to stick to the ring groove, which hinders its ability to transfer heat, seal the combustion chamber, and limit oil consumption.

Accordingly, to prevent the piston ring from sticking during prolonged use of the piston in the high temperature combustion engine, the first ring groove can be formed with a trapezoidal cross-section, also referred to as a keystone cross-section, rather than a conventional rectangular cross-section. A keystone piston ring having a matching trapezoidal cross-section is disposed in the keystone ring groove. The angled surfaces of the keystone ring groove and keystone piston ring prevent the piston ring from sticking. If the piston is designed for use in an engine application having even higher temperature loads, then the second ring groove is also formed with the keystone cross-section. However, use of the keystone ring groove is limited as it is significantly more expensive to manufacture than the conventional rectangular groove.

SUMMARY OF THE INVENTION

One aspect of the invention provides a piston for high temperature internal combustion engines. The piston includes an upper wall surrounding a center axis and presenting a combustion surface, an outer rib extending from the upper wall to a base wall, and an inner rib disposed between the outer rib and the center axis and extending from the upper wall to the base wall. The upper wall, ribs, and base wall present a cooling chamber therebetween. The outer rib presents an outer surface and includes a plurality of ring grooves. Each of the ring grooves includes a pair of side flanks extending inwardly from the outer surface to a base flank. The first ring groove, which is the ring groove closest to the combustion surface, has a rectangular cross-section. Thus, the side flanks of the first ring groove extend perpendicular to the base flank of the first ring groove. The second ring groove, which is the ring groove second closest to the combustion surface, has a keystone cross-section. Thus, at least one of the side flanks of the second ring groove is disposed at an angle greater than 90 degrees relative to the base flank of the second ring groove.

The piston of the present invention with the rectangular first ring groove and the keystone second ring groove is designed for engine applications wherein the temperature of the first ring groove exceeds 280° C. and thus is so hot that carbon does not deposit on the ring groove, or burns off. At such high temperatures, ring sticking is not an issue and thus the first ring groove does not require the keystone cross-section. However, in these high temperature engine applications, the temperature at the second ring groove is typically between 200° C. and 280° C. and thus carbon can deposit on the second ring groove causing the piston ring to stick. Accordingly, the second ring groove is formed with the keystone cross-section to prevent the piston ring from sticking. The piston can provide exceptional performance during operation of the high temperature combustion engine and can be formed with low manufacturing costs since the keystone cross-section is not formed in the first ring groove. In addition, the rectangular first ring groove provides improved sealing of combustion gases and oil along the cylinder wall, compared to pistons formed with a keystone first ring groove.

The invention also provides a method of manufacturing the piston. The method includes providing the piston with the upper wall, the base wall, the outer rib, and the inner rib presenting the cooling chamber therebetween. The method further includes forming the first ring groove and the second ring groove in the outer rib. This step includes forming each ring groove with the pair of side flanks extending inwardly from the outer surface to the base flank, wherein the side flanks of the first ring groove extend perpendicular to the base flank, and at least one of the side flanks of the second ring groove extend at an angle greater than 90 degrees relative to the base flank.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a side cross-sectional view of a piston including an upper part welded to a lower part according to one exemplary embodiment of the invention;

FIG. 1A is an enlarged view of the ring grooves of the piston of FIG. 1 without piston rings disposed in the ring grooves;

FIG. 2 is a side cross-sectional view of a single-piece cast piston according to another exemplary embodiment of the invention; and

FIG. 2A is an enlarged view of the ring grooves of the piston of FIG. 2.

DESCRIPTION OF THE ENABLING EMBODIMENT

Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, an exemplary piston 20, 120 for a high temperature internal combustion engine is generally shown in FIGS. 1 and 2. The piston 20, 120 includes an upper wall 22, 122, a base wall 24, 124, an outer rib 26, 126, and an inner rib 28, 128 defining a cooling chamber 30, 130 extending circumferentially around a center axis A₁. A plurality of ring grooves 32, 132, 34, 134, 36, 136, are formed in an outer surface 38, 138 of the outer rib 26, 126 each for retaining a piston ring 39, 139, 41, 141. During engine operation, the piston temperature at the first ring groove 32, 132 can exceed 280° C. and thus so hot that carbon does not deposit on the first ring groove 32, 132, or burns off. Accordingly, the first ring groove 32, 132 does not require a keystone cross-section and is formed with a conventional rectangular cross-section. The temperature of the piston 20, 120 at the second ring groove 34, 134, however, can be between 200° C. and 280° C., in which case carbon deposits can form on the second ring groove 34, 134. Thus, the second ring groove 34, 134 is formed with the keystone cross-section to prevent the piston ring 41, 141 from sticking. Preferably, only the second ring groove 34, 134 is formed with the keystone cross-section. Thus, the piston 20, 120 of the present invention can provide exceptional performance and can be formed with reduced manufacturing costs compared to pistons formed with the keystone first and keystone second ring grooves, and in some cases even more than two keystone ring grooves.

The piston 20, 120 of the present invention can comprise various different designs. For example, the piston 20, 120 can be a two-piece welded piston, such as a friction welded, laser welded, electron beam welded, or induction welded piston. The piston 20, 120 can also be a single-piece piston 20, 120, such as a cast piston. However, each piston design includes the rectangular first ring groove 32, 132 and the keystone second ring groove 34, 134.

The exemplary piston 20 of FIG. 1 is a friction-welded heavy duty piston including an upper piston part 40 friction welded to a lower piston part 42. The upper piston part 40 is formed of a metal material, such as steel, cast iron, aluminum, or another type of metal material. The upper piston part 40 extends annularly around the center axis A₁ and longitudinally along the center axis A₁ from the upper wall 22 to a first upper joining surface and a second upper joining surface. The upper wall 22 of the upper piston part 40 includes a combustion surface 44 presenting a bowl rim extending annularly around the center axis A₁ and a combustion bowl extending inwardly and downwardly from the bowl rim toward the center axis A₁. The combustion surface 44 of the upper wall 22 presents an apex at the center axis A₁ and is surrounded by the combustion bowl. During operation, the piston 20 reciprocates in a cylinder of the engine, and the combustion surface 44 together with the cylinder wall forms a combustion chamber containing a high temperature mixture of fuel and gas for ignition.

The upper piston part 40 includes an upper outer rib depending from the bowl rim of the upper wall 22 and extending annularly around the center axis A₁ and longitudinally along the center axis A₁ to the first upper joining surface. The upper outer rib presents an upper outer surface extending annularly around the center axis A₁ and facing away from the center axis A₁. The upper out rib forms a portion of the outer rib 26 of the finished friction-welded piston 20, and the upper outer surface forms a portion of the outer surface 38 of the finished friction-welded piston 20.

The upper piston part 40 also includes an upper inner rib disposed between the upper outer rib and the center axis A₁. The upper inner rib depends from the upper wall 22 beneath the combustion bowl and extends annularly around the center axis A₁ and longitudinally along the center axis A₁ to the second upper joining surface. The upper inner rib forms a portion of the inner rib 28 of the finished friction-welded piston 20. In the exemplary embodiment of FIG. 1, the first and second upper joining surfaces are offset from one another and each present a flat surface extending perpendicular to the center axis A₁. However, the upper ribs and the associated joining surfaces could present other geometries.

The lower piston part 42 of the friction-welded piston 20 is also formed of a metal material, which can be the same or different from the metal material of the upper piston part 40. The lower piston part 42 also extends annularly around the center axis A₁ and longitudinally along the center axis A₁. In the exemplary piston 20 of FIG. 1, the lower piston part 42 provides the base wall 24 which extends perpendicular to and surrounds the center axis A₁. A lower outer rib extends annularly around the center axis A₁ and upwardly from the base wall 24 to a first lower joining surface. The first lower joining surface of the lower piston part 42 and the first upper joining surface of the upper piston part 40 include a weld 46 therebetween. The lower outer rib also presents a lower outer surface extending annularly around the center axis A₁. The lower outer surface is aligned with the upper outer surface, and together those surfaces form the outer surface 38 of the finished friction-welded piston 20.

The lower piston part 42 also includes a lower inner rib disposed radially inwardly of the lower outer rib and spaced from the lower outer rib by the base wall 24. The lower inner rib extends annularly around the center axis A₁ and upwardly from the base wall 24 to a second lower joining surface. The second lower joining surface of the lower piston part 42 and the second upper joining surface of the upper piston part 40 also include the weld 46 therebetween. The lower inner rib and the upper inner rib together form the inner rib 28 of the finished friction-welded piston 20. In the exemplary embodiment of FIG. 1, the first and second lower joining surfaces are offset from one another and each present a flat surface extending perpendicular to the center axis A₁. However, the lower ribs and the associated joining surfaces could present other geometries.

As shown in FIG. 1, the welded inner rib 28, the welded outer rib 26, the upper wall 22, and the base wall 24 form the cooling chamber 30 therebetween. The cooling chamber 30 is closed and extends annularly around the center axis A₁. The cooling chamber 30 can be filled with a cooling fluid 43, such as oil, for transferring heat away from the upper wall 22 as the piston 20 reciprocates in the cylinder of the engine.

The welded inner rib 28 of the piston 20 surrounds the center axis A₁ and forms a cooling gallery 48 at the center axis A₁. In the exemplary embodiment of FIGS. 1 and 2, the cooling gallery 48 is closed along the upper wall 22 and opens in a direction facing away from the upper wall 22. Oil can be sprayed through the cooling gallery 48 to the underside of the upper wall 22 to further cool the piston 20. Alternatively, the cooling gallery 48 can be closed and thus no oil is sprayed into the cooling gallery 48.

The lower piston part 42 of the exemplary piston 20 of FIG. 1 also includes a pair of pin bosses 50 extending downwardly from the base wall 24 away from the upper piston part 40. Each of the pin bosses 50 presents a pin bore 52. The pin bores 52 are aligned with one another along a second axis A₂ which is perpendicular to the center axis A₁. The lower piston part 42 also includes a pair of skirt sections 54 each depending from the base wall 24 and spaced from one another by one of the pin bosses 50.

As shown in FIG. 1, the outer upper and lower ribs of the friction-welded piston 20 together provide the outer rib 28 with three ring grooves 32, 34, 36 formed therein. Preferably, no more than three ring grooves 32, 34, 36 are formed in the outer rib 28. Each of the ring grooves 32, 34, 36 extends radially inwardly from the outer surface 38 toward the center axis A₁ and circumferentially around the center axis A₁ for retaining a piston ring 39, 41. The three ring grooves 32, 34, 36 are axially aligned and spaced from one another by the outer surface 38 of the outer rib 26. All three ring grooves 32, 34, 36 are also preferably disposed axially along and radially outwardly of the cooling chamber 30. Each ring groove 32, 34, 36 includes a pair of side flanks 56 extending inwardly from the outer surface 38 to a base flank 58, as best shown in FIG. 1A.

The three ring grooves 32, 34, 36 formed in the outer rib 26 of the piston 20 of FIG. 1 include the first ring groove 32, which is the ring groove located closest to the combustion surface 44, having the rectangular cross-section. The side flanks 56 of the first ring groove 32 extend perpendicular to the base flank 58 to retain the piston ring 39 having a matching rectangular cross-section.

The second ring groove 34 formed in the outer rib 26, which is the ring groove located second closest to the combustion surface 44 and disposed between the first ring groove 32 and a third ring groove 36, has the keystone cross-section. Both side flanks 56 of the second ring groove 34 are disposed at an angle α greater than 90 degrees and not greater than 135 degrees relative to the base flank 58 for retaining the piston ring 41 having a matching keystone cross-section. Alternatively, the second ring groove 34 could have a semi-keystone cross-section, wherein only one of the side flanks 56 is disposed at the angle α greater than 90 degrees and not greater than 135 degrees relative to the base flank 58, while the other side flank 56 is disposed at an angle α equal to 90 degrees relative to the base flank 58. In the embodiment shown in FIGS. 1 and 1A, the upper side flank 56 and the base flank 58 of the second ring groove 34 are provided by the upper piston part 40 and the lower side flank 56 is provided by the lower piston part 42.

The third ring groove 36 shown in FIG. 1, which is the ring groove farthest from the combustion surface 44 and disposed between the second ring groove 34 and the base wall 24, also has the rectangular cross-section. The side flanks 56 of the third ring groove 36 extend perpendicular to the base flank 58 for retaining the piston ring 39 having a matching rectangular shape. Although the exemplary piston 20 of FIG. 1 is formed with three ring grooves 32, 34, 36, the piston 20 could alternatively include two ring grooves or more than three ring grooves, wherein the side flanks 56 of all of the ring grooves, except for the second ring groove 34, extend perpendicular to the base flank 58. Preferably, the second ring groove 34 is the only ring groove formed with the keystone cross-section.

As shown in FIGS. 1 and 1A, the base flanks 58 of the ring grooves 32, 34, 36 are preferably axially aligned with one another and extend parallel to the center axis A₁ and parallel to the outer surface 38 of the piston 20. As best shown in FIG. 1A, the base flanks 58 have a length L extending parallel to the center axis A₁ and from one of the side flanks 56 to the other side flank 56. In this exemplary embodiment, the length L₃ of the base flank 58 of the third ring groove 36 is greater than the length L₂ of the base flank 58 of the second ring groove 34, and greater than the length L₁ of the base flank 58 of the first ring groove 32. Each base flank 58 is preferably spaced from the cooling chamber 30 by a distance of not greater than three times the length L of the base flank 58.

Also best shown in FIG. 1A, the outer surface 38 of the piston 20 presents a first land extending a first distance d₁ from the combustion surface 44 to the first ring groove 32, a second land extending a second distance d₂ from the first ring groove 32 to the second ring groove 34, and a third land extending a third distance d₃ from the second ring groove 34 to the third ring groove 36. The first distance d₁ is preferably not greater than five times the length L₁ of the base flank 58 of the first ring groove 32. The second distance d₂ and the third distance d₃ are preferably not greater than three times the length L₁ of the base flank 58 of the first ring groove 32.

FIG. 2 illustrates the piston 120 of the present invention having an alternative design, but still including the rectangular first ring groove 132 and the keystone second ring groove 134. This piston 120 comprises a single-piece body formed by casting the metal material. The piston 120 includes the upper wall 122 presenting the combustion surface 144 with the bowl rim extending annularly around the center axis A₁ and the combustion bowl extending inwardly and downwardly from the bowl rim toward the center axis A₁. The combustion surface 144 of the upper wall 122 presents the apex at the center axis A₁ and is surrounded by the combustion bowl, like the piston 120 of FIG. 1.

The piston 120 of FIG. 2 also includes the outer rib 126 depending from the bowl rim of the upper wall 122 and extending annularly around the center axis A₁ and longitudinally along the center axis A₁ to the base wall 124. The outer rib 126 presents the outer surface 138 of the piston 120 with the ring grooves 132, 134, 136. The piston 120 also includes the inner rib 128 disposed between the outer rib 126 and the center axis A₁. The inner rib 128 depends from the upper wall 122 beneath the combustion bowl and extends annularly around the center axis A₁ and longitudinally along the center axis A₁ to the base wall 124. The inner rib 128, outer rib 126, upper wall 122, and base wall 124 together form the cooling chamber 130 therebetween. In this embodiment, the upper wall 122 forms the cooling gallery 148 at the center axis A₁. The piston 120 also includes the pair of pin bosses 150 extending downwardly from the base wall 124 and each presenting a pin bore 152, and the skirt sections 154 each depending from the base wall 124 and spaced from one another by one of the pin bosses 150.

As shown in FIG. 2, the outer rib 126 of the single-piece cast piston 120 includes the three ring grooves 132, 134, 136, and no more than three ring grooves. Each of the ring grooves 132, 134, 136 extends radially inwardly from the outer surface 138 toward the center axis A₁ and circumferentially around the center axis A₁ for retaining a piston ring 139, 141. The three ring grooves 132, 134, 136 are axially aligned and spaced from one another by the outer surface 138 of the outer rib 126. All three ring grooves 132, 134, 136 are preferably disposed axially along and radially outwardly of the cooling chamber 130. As best shown in FIG. 2A, each ring groove 132, 134, 136 includes the side flanks 156 extending inwardly from the outer surface 138 to the base flank 158. Like the piston 120 of FIG. 1, the first ring groove 132 and the third ring groove 136 have the rectangular cross-section, and the second ring groove 134 is the only ring groove with the keystone cross-section. Although not shown, the piston 120 could alternatively include only the first and second ring grooves 132, 134, or more than three ring grooves, wherein the second ring groove 134 is the only ring groove with the keystone cross-section.

Another aspect of the invention provides a method of forming the piston 20, 120 including the rectangular first ring groove 32, 132 and the keystone second ring groove 34, 134. The method first includes providing the piston 20, 120 including the upper wall 22, 122 surrounding the center axis A₁ and presenting the combustion surface 44, 144; the outer rib 26, 126 extending from the upper wall 22, 122 to the base wall 24, 124 and presenting the outer surface 138, 138; and the inner rib 28, 128 extending from the upper wall 22, 122 to the base wall 24, 124 and disposed between the outer rib 26, 126 and the center axis A₁; and wherein the upper wall 22, 122, the ribs 26, 126, 28, 128 and the base wall 24, 124 present the cooling chamber 30, 130 therebetween. To form the two-piece piston 20 of FIG. 1, this first step includes providing the upper piston part 40 and the lower piston part 42, and then welding the upper piston part 40 to the lower piston part 42. To form the single-piece piston 120 of FIG. 2, this first step includes casting the metal material.

The method further includes forming the plurality of ring grooves 32, 132, 34, 134, 36, 136 in the outer rib 26, 126. This step can be performed by machining, for example after welding the upper piston part 40 to the lower piston part 42, or after casting the body of the piston 120. Alternatively, the ring grooves 132, 134, 136 can be formed during the casting process.

The step of forming the ring grooves 32, 132, 34, 134, 36, 136 includes forming each ring groove with the side flanks 56, 156 extending inwardly from the outer surface 38, 138 to the base flank 58, 158. This step also includes forming at least one of the side flanks 56, 156 of the second ring groove 34, 134 to extend at the angle α greater than 90 degrees and not greater than 135 degrees relative to the base flank 58, 158 of the second ring groove 34, 134. Preferably, the side flanks 56, 156 of the first ring groove 32, 132 and the side flanks 56, 156 of all other ring grooves except for the keystone second ring groove 34, 134 are formed to extend perpendicular to the base flank 58, 158. The method also preferably includes forming not more than three of the ring grooves 32, 132, 34, 134, 36, 136 in the outer rib 26, 126.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the appended claims. 

What is claimed is:
 1. A piston, comprising: an upper wall surrounding a center axis and presenting a combustion surface; an outer rib extending from said upper wall to a base wall and presenting an outer surface; an inner rib disposed between said outer rib and said center axis and extending from said upper wall to said base wall; said upper wall and said ribs and said base wall presenting a cooling chamber therebetween; said outer rib including a plurality of ring grooves; said plurality of ring grooves including a first ring groove being the ring groove closest to said combustion surface and a second ring groove being the ring groove second closest to said combustion surface; each of said ring grooves including a pair of side flanks extending inwardly from said outer surface to a base flank; said side flanks of said first ring groove extending perpendicular to said base flank of said first ring groove; and at least one of said side flanks of said second ring groove being disposed at an angle greater than 90 degrees relative to said base flank of said second ring groove.
 2. The piston of claim 1, wherein said plurality of ring grooves include a third ring groove being the ring groove third closest to said combustion surface; and said side flanks of said third ring groove extend perpendicular to said base flank of said third ring groove.
 3. The piston of claim 2 including not more than three of said ring grooves in said outer rib.
 4. The piston of claim 1, wherein said side flanks of all of said ring grooves except said second ring groove extend perpendicular to said base flank.
 5. The piston of claim 1, wherein said side flank of said second ring groove closest to said combustion surface is disposed at an angle greater than 90 degrees and not greater than 135 degrees relative to said base flank of said second ring groove.
 6. The piston of claim 5, wherein both of said side flanks of said second ring groove are disposed at an angle greater than 90 degrees and not greater than 135 degrees relative to said base flank of said second ring groove.
 7. The piston of claim 1, wherein said ring grooves are axially aligned with one another and axially spaced from one another by said outer surface of said outer rib; and each of said ring grooves extends radially inwardly from said outer surface toward said center axis and circumferentially around said center axis.
 8. The piston of claim 1, wherein each of said ring grooves is disposed axially along and radially outwardly of said cooling chamber.
 9. The piston of claim 1 including an upper piston part and a lower piston part welded together and together forming said outer rib, wherein one of said side flanks and said base flank of said second ring groove is provided by said upper piston part and the other side flank of said second ring groove is provided by said lower piston part.
 10. The piston of claim 1, wherein said base flanks of said ring grooves extend parallel to said center axis and parallel to said outer surface; said base flanks are axially aligned with one another; each of said base flanks has a length extending parallel to said center axis from one of said side flanks to the other side flank of said ring groove; each of said base flanks is spaced from said cooling gallery by a distance of not greater than three times the length of said base flank; said outer surface presents a first distance between said combustion surface and said first ring groove and a second distance between said first ring groove and said second ring groove and a third distance between said second ring groove and said third ring groove; said first distance is not greater than five times the length of said base flank of said first ring groove; and said second distance and said third distance are each not greater than three times the length of said base flank of said first ring groove.
 11. The piston of claim 1, wherein said first ring groove is at a temperature greater than 280° C. and said second ring groove is at a temperature between 200° C. and 280° C. during operation of said piston in an internal combustion engine.
 12. A piston, comprising: an upper piston part formed of a metal material and extending annularly around a center axis and longitudinally along said center axis from an upper wall to a first upper joining surface and a second upper joining surface; said metal material of said upper piston part being a steel, cast iron, or aluminum material; said upper wall of said upper piston part including a combustion surface presenting a bowl rim extending annularly around said center axis and a combustion bowl extending inwardly and downwardly from said bowl rim toward said center axis; said combustion surface of said upper wall presenting an apex at said center axis and surrounded by said combustion bowl; said upper piston part including an upper outer rib depending from said bowl rim of said upper wall and extending annularly around said center axis and longitudinally along said center axis to said first upper joining surface; said upper outer rib presenting an upper outer surface extending annularly around said center axis and facing away from said center axis; said upper piston part including an upper inner rib disposed between said upper outer rib and said center axis; said upper inner rib depending from said upper wall beneath said combustion bowl and extending annularly around said center axis and longitudinally along said center axis to said second upper joining surface; said first and second upper joining surfaces being flat and extending perpendicular to said center axis; a lower piston part formed of a metal material and extending annularly around said center axis and longitudinally along said center axis; said metal material of said lower piston part being steel, cast iron, or aluminum material; said lower piston part including a lower outer rib extending from a base wall to a first lower joining surface, said first lower joining surface and said first upper joining surface including a weld therebetween; said base wall extending perpendicular to and surrounding said center axis said lower outer rib extending annularly around said center axis and upwardly from said base wall to said first lower joining surface; said lower outer rib presenting a lower outer surface extending annularly around said center axis and aligned with said upper outer surface; said lower piston part including a lower inner rib spaced from said lower outer rib by said base wall and disposed radially inwardly of said lower outer rib; said lower inner rib extending from said base wall to a second lower joining surface, said second lower joining surface and said second upper joining surface including a weld therebetween; said lower inner rib extending annularly around said center axis and upwardly from said base wall to said second lower joining surface; said first and second lower joining surfaces being flat and perpendicular to said center axis; said welded inner ribs and said welded outer ribs and said upper wall and said base wall forming a cooling chamber therebetween; said cooling chamber being closed and extending annularly around said center axis; said inner ribs surrounding said center axis and forming a cooling gallery at said center axis; said cooling gallery being closed along said upper wall and open in a direction facing away from said upper wall; said lower piston part including a pair of pin bosses extending downwardly from said base wall away from said upper piston part; each of said pin bosses presenting a pin bore, said pin bores being aligned with one another along a second axis perpendicular to said center axis; said lower piston part including a pair of skirt sections each depending from said base wall and spaced from one another by one of said pin bosses; said outer ribs together including three ring grooves each extending radially inwardly from said outer surface toward said center axis and circumferentially around said center axis for retaining a piston ring; said outer ribs together including no more than three ring grooves; said three ring grooves including a first ring groove and a second ring groove and a third ring groove being axially aligned and spaced from one another by said outer surface of said outer ribs; said first ring groove being the ring groove closest to said combustion surface; said second ring groove being the ring groove second closest to said combustion surface and disposed between said first ring groove and said second ring groove; said third ring groove being the ring groove farthest from said combustion surface and being disposed between said second ring groove and said base wall; each of said ring grooves being disposed axially along and radially outwardly of said cooling chamber; each of said ring grooves including a pair of side flanks extending inwardly from said outer surface to a base flank; said side flanks of said first ring groove extending perpendicular to said base flank of said first ring groove for retaining a piston ring having a rectangular shape; said side flanks of said third ring groove extending perpendicular to said base flank of said third ring groove for retaining a piston ring having a rectangular shape; at least one of said side flanks of said second ring groove being disposed at an angle greater than 90 degrees relative to said base flank of said second ring groove for retaining a piston ring having a keystone shape; one of said side flanks and said base flank of said second ring groove being provided by said upper piston part and the other one of said side flanks of said second ring groove being provided by said lower piston part; said base flanks of said ring grooves being axially aligned with one another and extending parallel to said center axis and parallel to said outer surface; each of said base flanks having a length extending parallel to said center axis from one of said side flanks to the other of said side flanks, the length of said base flank of said third ring groove being greater than the length of said base flank of said second ring groove and greater than the length of said base flank of said first ring groove; each of said base flanks being spaced from said cooling chamber by a distance of not greater than three times the length of said base flank; said outer surface presenting first land extending a first distance from said combustion surface to said first ring groove, a second land extending a second distance from said first ring groove to said second ring groove, and a third land extending a third distance from said second ring groove to said third ring groove; said first distance being no greater than five times the length of said base flank of said first ring groove; and said second distance and said third distance being no greater than three times the length of said base flank of said first ring groove.
 13. A method of forming a piston, comprising the steps of: providing a piston including an upper wall surrounding a center axis and presenting a combustion surface, an outer rib extending from the upper wall to a base wall and presenting an outer surface, and an inner rib extending from the upper wall to the base wall and disposed between the outer rib and the center axis, the upper wall and the ribs and the base wall presenting a cooling chamber therebetween; and forming a plurality of ring grooves in the outer rib; the step of forming the ring grooves including forming a first ring groove being the ring groove closest to the combustion surface and a second ring groove being the ring groove second closest to the combustion surface; and the step of forming the ring grooves including forming each ring groove with a pair of side flanks extending inwardly from the outer surface to a base flank, the side flanks of the first ring groove extending perpendicular to the base flank of the first ring groove, and at least one of the side flanks of the second ring groove extending at an angle greater than 90 degrees relative to the base flank of the second ring groove.
 14. The method of claim 13, wherein the step of forming the ring grooves includes forming a third ring groove being the ring groove farthest from the combustion surface; and forming the third ring groove with side flanks extending perpendicular to the base flank of the third ring groove.
 15. The method of claim 13, wherein the step of forming the ring grooves includes forming not more than three of the ring grooves in the outer rib.
 16. The method of claim 13, wherein the step of forming the ring grooves includes forming side flanks of all of the ring grooves extending perpendicular to the base flank except for the at least one side flank of the second ring groove.
 17. The method of claim 13, wherein the step of forming the ring grooves includes forming the side flank of the second ring groove closest to the combustion surface at an angle greater than 90 degrees and not greater than 135 degrees relative to the base flank of the second ring groove.
 18. The method of claim 17 including forming both of the side flanks of the second ring groove an angle greater than 90 degrees and not greater than 135 degrees relative to the base flank of the second ring groove.
 19. The method of claim 13, wherein the step of forming the ring grooves includes machining or casting.
 20. The method of claim 13 including providing an upper piston part and a lower piston part, and welding the upper piston part to the lower piston part, wherein each of the piston parts provide a portion of the outer rib and a portion of the inner rib. 